This invention relates to removing alkyl lead from gaseous streams, and particularly relates to reducing the concentration of alkyl lead that is present in process air streams of a tetralkyl lead plant.
In a widely used commercial process for the preparation of tetralkyl lead, a lead-sodium alloy is reacted in an autoclave with an alkyl halide, usually methyl chloride or ethyl chloride. Much of the lead remains unreacted in the product in the form of a sludge. The product is steam distilled for recovery of the alkyl lead, and the sludge residue is then sent to a holding pit where some sodium chloride formed in the reaction is washed out. The remainder is dried and purified in a lead smelting furnace. The purified lead is made into a lead-sodium alloy and recycled for further reaction with alkyl halide.
In the smelting furnace, the washed sludge residue is simply melted down, forming a pool of molten lead covered with slag. In a typical reverberatory smelting furnace, flames from forced air burners are directed over the sludge at temperatures typically between about 590.degree. C. to about 810.degree. C. This creates flue gases containing substantial amounts of dust, which must be removed before the gas is vented to the atmosphere. Therefore, the gas is passed through bag filters where particulate lead compounds, dust, and other solid particles are removed and from which substantially particle free gas is vented. Other devices can be used to remove the dust, such as cyclones and electrostatic precipitators.
In a process of manufacturing a toxic substance such as tetraalkyl lead, such as described above, great care is taken to avoid exposure of workers to harmful concentrations that may be unavoidably or inadvertantly released to their environment. As a result, ventilating systems of very high capacity are used to keep the tetraalkyl lead content of the air in the manufacturing buildings at safe levels. If the contaminated ventilating air is passed directly to the atmosphere, even though the lead concentration in the vented air is low, the volume of air is so great that a significant amount of lead may be emitted. From a tetraalkyl lead plant of substantial size, undesirable amounts of lead may be lost each day in this way.
U.S. Pat. Nos. 3,403,495 and 3,413,328 describe the recovery of tetraalkyl lead from gaseous streams by scrubbing with an inert organic liquid. These processes are well adopted to recovery of tetraalkyl lead from streams of relatively small volume and relatively high concentration, but they become less pratical as the process stream volume increases and the concentration of the tetraalkyl lead in the contaminated stream decreases.